One of the main areas of expertise of the footwear industry is good control of the adhesive bonding techniques intended to assemble materials of different chemical natures and with different mechanical properties. This expertise is particularly important in the field of sports shoes, where the materials used, in particular for the manufacture of the soles, are frequently novel materials. This requirement is magnified by the search for performance generally related to the sports shoe.
During the last decade, materials based on TPE, such as the PEBA materials sold by Arkema under the Pebax® trademark, have gradually become established in the field of top of the range footwear, in particular sports shoes, by virtue of their mechanical properties and in particular their exceptional resilience property. Specifically, PEBAs can advantageously be used in sports shoes as sole of “semi-rigid” type (football, baseball, and the like) or flexible type (jogging), making it possible to directly produce the internal sole (damping) and/or the external sole (abrasion resistance-stiffness).
The assembling of TPE on TPEs of the same nature or with different compositions is obtained by moulding or extruding, possible cutting up of the components and then adhesively bonding and pressing these components, or also by direct adhesion of these TPEs.
The term “direct adhesion process” is understood to mean an adhesion process without contribution of binder, in particular contribution of adhesive. In comparison with conventional adhesive bonding processes involving a multitude of complicated stages generally using adhesives based on organic solvents, direct adhesion processes are more ecological and non-polluting.
Mention may be made, as examples of direct adhesion process, of: overmoulding, hot pressing, coextrusion, thermoforming, two-material injection moulding, co-injection moulding and any other possible adhesion method using one or more of the conventional methods, such as injection moulding, extrusion moulding and/or blow moulding.
This cohesion between the materials is rendered possible by the adhesive properties which the polymers possess in the molten state. For example, the patent document EP197845 describes a process for assembling surfaces by application of a PEBA seal and melting of its surface layer. The patent document EP179700 describes an adhesive composition comprising a PEBA and its use as pressure-sensitive adhesive. The patent document EP0679119 describes a two-layer object comprising a lightened thermoplastic elastomer of polyetheramide type which adheres by itself to a non-lightened thermoplastic which can be chosen from polyetheramides, polyetheresters or polyurethanes.
The overmoulding technique consists of the injection of substance over an insert placed at the bottom of the mould. The cohesion of the two materials is obtained by the hot melt and compatibility properties of the overmoulded substance and of the insert. PEBAs are very well suited to this overmoulding technique as they have a broad plasticizing range, making it possible to optimize the adhesion of the substance and of the inserts and to avoid the use of adhesive.
Unfortunately, the levels of adhesion, expressed by the peel strength, of the TPE-based substrates of the systems of the prior art obtained by direct adhesion are far from being optimal. Thus, with some of the PEBA substrates with a Shore D hardness of 60 to 65 on average (for example Pebax® 6333), at best weak peel strengths of 3 kg/cm are obtained. In point of fact, footwear manufacturers require, for some applications, a peel strength of at least 5, preferably of at least 8, indeed even of at least 10 kg/cm.
It is thus an aim of the present invention to provide a PEBA with properties of improved direct adhesion to TPE substrates and in particular to TPU.
Another aim of the present invention is to provide a laminate comprising at least one PEBA-based substrate and also a process for the manufacture of such a laminate by direct adhesion, the laminate having to exhibit a peel strength of greater than 3 kg/cm, preferably of at least 5 kg/cm, preferably of at least 8 kg/cm, indeed even of at least 10 kg/cm, measured according to Standard ISO 11339.
A further aim of the present invention is to increase the level of adhesion of TPE materials without detrimentally affecting their mechanical properties.
The Applicant Company has now developed a novel type of PEBA which exhibits a markedly better direct adhesion to TPE than that of the existing PEBAs, with a peel strength of greater than 10 kg/cm, while retaining excellent mechanical properties, such as good liveliness, a low density and low cold stiffening.